HONG KONG | China’s notorious organized crime groups known as triads, which once thrived in collaboration with communist clandestine services, still operate large-scale criminal enterprises but are facing pressure from local authorities.

Hong Kong police recently rounded up a network of triads by using an undercover operative who infiltrated one group in Kowloon, across from Hong Kong Island, a police spokesman said.

Police arrested 33 men, ages 16 to 55, including a senior triad leader. They were charged with offenses ranging from membership in an organized crime group and assault to criminal damage and illegal drug possession.

“During the operation, police also seized a small [cache] of drugs and offensive weapons,” police spokesman T.K. Ng said in an e-mail. “The aim of this operation is to curb triad activities and their sources of income.”

Acting Police Superintendent Lam Sai-kit of the Organized Crime and Triad Bureau told the South China Morning Post that the triad network was recruiting youths and engaged in extortion, assault and drug trafficking.

The undercover operation, called “Eventwatcher,” led to the identification of at least 50 members of the triads. The raid included the arrest of a former “dragonhead,” as leaders of the group are called, named Hak Chai Ka. (Hak Chai means “bad luck” in Cantonese.)

Triads have existed for hundreds of years in China, composed initially of rebels who opposed Manchu rule and eventually branching out in the mid-1700s into several groups, including the Three Harmonies Society, which used a triangle as its emblem. British authorities in colonial Hong Kong dubbed the groups triads because of the triangular imagery.

Like other organized crime groups, triads have elaborate initiation ceremonies similar to those of the Italian Mafia and are engaged in a range of illegal activities such as bank and credit card fraud, currency counterfeiting, money laundering, extortion, human trafficking and prostitution.

U.S. officials have said Chinese triads were linked to the trafficking of North Korean-made, high-quality counterfeit $100 bills, including an operation that led to arrests in Las Vegas last year.

Hong Kong’s Communist Party unit, which Beijing operated underground for decades and remains largely behind the scenes today, has had close ties to the triads.

“The triads have always worked with the Chinese Communist Party since its inception in the early 1920s and continue to do so,” said Brian McAdam, a retired Canadian diplomat who specializes in Chinese organized crime.

Mr. McAdam estimates the number of triad members in Hong Kong at between 150,000 and 250,000 people.

In the U.S., congressional investigators in the 1990s identified “China-gate” fundraising scandal figure Charlie Trie as a triad member who aided a Chinese government campaign to funnel millions of dollars to the Democratic Party. Trie was traced by investigators to Ng Lap Seng, a Macau-based triad.

Sun Yat Sen, the founder of modern China, had links to triads, and Communist leader Zhou Enlai worked with Shanghai triads to recruit intelligence agents and kill traitors, Mr. McAdam said.

In addition, Zhu De, founder of China’s People’s Liberation Army, was once a member of a triad, and Kang Sheng, China’s first intelligence chief, also worked with “patriotic” triads, he said.

Mr. McAdam said that in 1993, Tao Siju, chief of China’s Public Security Bureau, gave an informal press conference to a group of television reporters in Hong Kong and was quoted as saying: “As for organizations like the triads in Hong Kong, as long as these people are patriotic, as long as they are concerned with Hong Kong’s prosperity and stability, we should unite with them.” Mr. Tao also invited the triads to set up businesses in China.

A powerful triad leader in Hong Kong was among the first to begin money laundering on behalf of China, converting billions of dollars worth of criminal money that was funneled to China and funding the Chinese military, said Mr. McAdam, who is writing a book on the subject.

Today, Hong Kong remains one of the money-laundering capitals of the world. Authorities in Beijing announced this week they are moving to curb the transfer of cash by limiting to $8,000 the amount of U.S. currency mainlanders can bring into Hong Kong.

A Hong Kong Communist official disclosed in 1997, the year the city state reverted from British rule to Chinese control, that the Communists worked closely with the triads to help the transition of power.

Wong Man-fong, a former official of the New China News Agency, the Chinese government organization in Hong Kong before 1997, was quoted in local press as saying that he had befriended triad leaders before the signing of the 1994 agreement with Britain that transferred sovereignty to Beijing.

“I told them that what the administration wanted was a peaceful return and that they could not attempt to do anything to upset Hong Kong’s prosperity and stability,” Mr. Wong said, noting that in return China pledged to ignore triad activities.

“I made the point of warning [triad leaders] that the Communist Party was not someone they wanted to mess with,” Mr. Wong was quoted as telling local press.

During the colonial period, China used Community Party front groups, such as state-run news agencies and the Party’s United Front Work Department, to operate underground in Hong Kong.

A U.S. official familiar with Chinese affairs confirmed that the triads have had a long relationship with China’s Communists who used the groups to obtain embargoed goods for China’s military and civilian industrialization. “They are very secret and difficult to find,” the official said.